The present invention relates to a receiver having an automatic gain controller, and more particularly, to an automatic gain controller for use in primarily receiving digital modulation signals (e.g. OFDM signals, 8VSB signals, 64QAM signals, QPSK signals) transmitted in a terrestrial broadcast, a CATV broadcast, a satellite broadcast, etc.
Recently, a digital broadcast by the satellite wave, terrestrial or cable has been performed.
A receiver for demodulating digital modulation broadcast signals is constructed to supply a received radio frequency signal (hereinafter, referred to as RF signal) to a mixer via a first variable gain circuit. In the mixer the RF signal and a local oscillation signal generated at a local oscillator are mixed together, and thus an intermediate frequency signal (hereinafter, referred to as IF signal) is output therefrom.
The IF signal output from the mixer is amplified in an IF-amplifier and then supplied an A/D converter via a second variable gain circuit. Thus the IF signal is converted to a digital IF signal at the A/D converter and after that the digital IF signal is demodulated at a digital demodulator.
In general, the first variable gain circuit is referred to as an RF AGC circuit, while the second variable gain circuit is referred to as an IF AGC circuit. These AGC circuits are constructed to be controlled their gains in response to the levels of the digital IF signal. Thus, the IF signal passing through the AGC circuits is adjusted to have an optimal level.
The RF signal which is input through the input terminal has normally a power in a range from about 90 dB (mW) to 10 dB (mW) per 1 channel. Since the first variable gain circuit, the mixer and the IF amplifier have the non-linear region in such an input level range, when the greater signal is input to the adjacent channel it causes the non-linear distortion. These non-linear distortions degrade the reception quality of the desired channel. So that these have to be reduced in the first variable gain circuits which are placed in relatively near to the input terminal.
On the other hand, when a relatively low level RF input signal is gain-controlled at only the first variable gain controllers that are close to the input terminal, the non-linear distortion can be improved better. However, it cannot keep a required C/N ratio, So that the reception quality is degraded. So, the lower input level signals have to be gain-controlled in the second variable gain control circuit which is remote from the input terminal, so as to keep the required C/N ratio.
Accordingly, when the RF input level to the receiver is lower than a take-over point (hereinafter, referred to as xe2x80x9cTOPxe2x80x9d) representing a reference level it activates the second variable gain circuit, and when the RF input level to the receiver is higher than the reference level it activates the first variable gain circuits. So, it is important to set the reference level optimal for decrease the non-linear distortion as keeping the required C/N ratio, and maintain the fine reception quality.
Here, Japanese paten application JP9-181832 which was laid-open on Jul. 11, 1997 discloses a tuner circuit comprising an RF AGC circuit and an IF AGC circuit which is capable of controlling AGC voltages to be applied to an RF amplifier and an IF amplifiers in response to levels of input RF signal.
However, conventionally in a representative receiving channel, the TOP value is setup that is receivable enough under the worst condition that there is an interference signal in the adjacent channel.
However, according to the conventional technique, since there are differences of efficiency in the non-linear distortion, the gain, and the noise index in the radio frequency circuit between the representative channel setup the TOP value and other channels, it is difficult to obtain the optimal reception quality in all receiving channel.
Further, there is another drawback that since the TOP value is setup under the worst condition that there is interference such as an adjacent-channel interference, it degrades the C/N ratio excessively when the interference level is lower or there is no interference.
As mentioned above, there is a drawback those since the TOP value is setup in the representative channel the relation between the degradation and the C/N ratio differs from the preferable value. Further, when the interference level is low it degrades the C/N excessively.
So, the present invention has been made in view of the problems shown above and it is the object of the present invention to provide an automatic gain controller and a receiver having the automatic gain controller which obtain the stable and fine reception quality without being affected by the receiving channel or receiving electric wave.
A first aspect of the automatic gain controller for use in a receiver for demodulating a radio frequency signal includes a circuit located on a passage of the radio frequency signal, which having a non-linear region to an input level of the radio frequency signal which is supplied as an input, a first variable gain circuit which is placed ahead the circuit with the non-linear region, a second variable gain circuit which is put after the circuit with the non-linear region, a gain controller, which is for controlling the gains of the first and the second variable gain circuits according to the signal level output from the second variable gain control, for controlling to activate the gain control of the second variable gain circuit when the input level of the radio frequency signal to the receiver is lower than a reference level, and controlling to activate the gain control of the first variable gain circuit and also controlling to maintain the gain of the second variable gain circuit to the certain level when the input level is over the reference level, a reception quality detector for detecting the reception quality which is defined after the second variable gain circuit, and a reference level setting unit for setting up the reference level, which is variable according to the reception quality detected in the reception quality detector, in the gain controller.
Further to the automatic gain controller as defined in the first aspect of the invention, in a second aspect of the automatic gain controller the reception quality of signals is detected first by a reference level pre-designated in the reference level setting unit, and again detected by another reference level which is varied from the pre-designated reference level in the direction that the reception quality is improved.
Further to the automatic gain controller as defined in the first or second aspect of the invention, in a third aspect of the automatic gain controller the reception quality detector detects the reception quality of signals by using a spread constellation.
Further to the automatic gain controller as defined in the first or second aspect of the invention, a fourth aspect of the automatic gain controller the reception quality detector detects the reception quality of signals by using an error correction ratio.
Further to the automatic gain controller as defined in the first or second aspect of the invention, a fourth aspect of the automatic gain controller the reception quality detector detects the reception quality of signals by using an error correction ratio.
Further to the automatic gain controller as defined in the first or second aspect of the invention, in a fifth aspect of the automatic gain controller the reception quality detector detects the reception quality by using an error ratio of signals before or after the error correction of the signals.
Further to the automatic gain controller as defined in any one of the first to fifth aspects of the invention, a sixth aspect of the automatic gain controller includes a memory for storing the values of the reference level, and wherein a reference level which is varied to optimally increase the reception quality of signals and thus capable to be used as an initial value of the reference signal in a following reception of signals.
An aspect of a receiver for demodulating a radio frequency according to the present invention signal includes an input terminal for receiving a radio frequency signal, a frequency converter for converting the radio frequency signal received at the input terminal into an intermediate frequency signal, a first variable gain circuit followed by the frequency converter, a second variable gain circuit placed in following the frequency converter, a gain controller, which is for controlling the gains of the first and the second variable gain circuits according to the signal level output from the second variable gain control, for controlling to activate the gain control of the second variable gain circuit when the input level of the radio frequency signal to the receiver is lower than a reference level, and controlling to activate the gain control of the first variable gain circuit and also controlling to maintain the gain of the second variable gain circuit to the certain level when the input level is over the reference level, a reception quality detector for detecting the reception quality which is defined after the second variable gain circuit, and a reference level setting unit for setting up the reference level, which is variable according to the reception quality detected in the reception quality detector, in the gain controller.
According to the present invention, it has a detector for detecting the reception quality, and a reference level (i.e., TOP) value for switching the operation between the second variable gain circuit (IF_AGC) and the first variable gain circuits (RF_AGC) is varied so as to obtain an optimal reference level. As a result, according to condition of the radio frequency which differs from each receiving channel, or the level relationship between adjacent channel signals, it determines the degradation of the reception quality caused by the non-linear distortion or the C/N ratio insufficiency so as to obtain an optimal receiving condition. The reception quality detector can use a spread constellation, an error correction ratio, or an error rate.
Additional objects and advantages of the present invention will be apparent to persons skilled in the art from a study of the following description and the accompanying drawings, which are hereby incorporated in and constitute a part of this specification.